Various types of membrane diffusion barriers are known according to the published prior art. Molecular and ionic species diffuse through such membranes according to the value of their diffusion coefficient, and for that reason their selectivity is generally poor. Also known according to the prior art are liquid membranes, immobilized liquid membranes or interfacial polymerization membranes which, whilst useful in certain specific applications, suffer the same defects as other previously known membranes.
Small molecular weight separations are not easily achieved by economical industrial stage processes. Classical hyperfiltration requires high pressure and high-equipment cost for the low selectivity, low fluxes and the always present risk of membrane fouling. Chromatography, including high pressure liquid chromatography, gives excellent selectivity but is limited to laboratory scale separations, and is a too costly operation for most industrial applications. Ion exchange, ionic exclusion, transport depletion and reverse electrodialysis are processes applicable only when macro-molecules, with isoelectric points different to the overall pH, are not present in large quantities, otherwise poisoning occurs. Furthermore, high investment cost and high operating cost prohibits their use for the treatment of cheap feedstocks for the fermentation industry.